Communications systems using optical signals require the use of optical switches and routers. An early approach to optical switching was to convert the optical signal to an electrical signal, use an electrical switch or router and then convert back to an optical signal. More recently, optical relays have been used in which an electrical control signal is used to control the switching or routing of an optical signal. Optical relays typically switch optical signals by using movable solid mirrors or by using the creation of vapor bubbles to alter the index of refraction inside a cavity. The moveable mirrors may use electrostatic latching mechanisms, whereas bubble switches do not latch. Piezoelectric latching relays either use residual charges in the piezoelectric material to latch, or actuate switch contacts containing a latching mechanism.
Liquid metal is also used in electrical relays. A liquid metal droplet can be moved by a variety of techniques, including electrostatic forces, variable geometry due to thermal expansion/contraction, and pressure gradients. When the dimension of interest shrinks, the surface tension of the liquid metal becomes dominant force over other forces, such as body forces (inertia). Consequently, some micro-electro-mechanical (MEM) systems utilize liquid metal switching.